Reverse link channel architecture for a wireless communication system

ABSTRACT

A channel structure and mechanisms that support effective and efficient allocation and utilization of the reverse link resources. In one aspect, mechanisms are provided to quickly assign resources (e.g., a supplemental channel) as needed, and to quickly de-assign the resources when not needed or to maintain system stability. The reverse link resources may be quickly assigned and de-assigned via short messages exchanged on control channels on the forward and reverse links. In another aspect, mechanisms are provided to facilitate efficient and reliable data transmission. A reliable acknowledgment/negative acknowledgment scheme and an efficient retransmission scheme are provided. Mechanisms are also provided to control the transmit power and/or data rate of the remote terminals to achieve high performance and avoid instability.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a Divisional and claims priorityto patent application Ser. No. 09/788,259 entitled “Reverse Link ChannelArchitecture for a Wireless Communication System” filed Feb. 15, 2001now U.S. Pat. No. 7,120,134, and assigned to the assignee hereof andhereby expressly incorporated by reference herein.

BACKGROUND

1. Field

The present invention relates generally to data communication, and morespecifically to a novel and improved reverse link architecture for awireless communication system.

2. Background

Wireless communication systems are widely deployed to provide varioustypes of communication including voice and packet data services. Thesesystems may be based on code division multiple access (CDMA), timedivision multiple access (TDMA), or some other modulation techniques.CDMA systems may provide certain advantages over other types of system,including increased system capacity.

In a wireless communication system, a user with a remote terminal (e.g.,a cellular phone) communicates with another user through transmissionson the forward and reverse links via one or more base stations. Theforward link (i.e., downlink) refers to transmission from the basestation to the user terminal, and the reverse link (i.e., uplink) refersto transmission from the user terminal to the base station. The forwardand reverse links are typically allocated different frequencies, amethod called frequency division multiplexing (FDM).

The characteristics of packet data transmission on the forward andreverse links are typically very different. On the forward link, thebase station usually knows whether or not it has data to transmit, theamount of data, and the identity of the recipient remote terminals. Thebase station may further be provided with the “efficiency” achieved byeach recipient remote terminal, which may be quantified as the amount oftransmit power needed per bit. Based on the known information, the basestation may be able to efficiently schedule data transmissions to theremote terminals at the times and data rates selected to achieve thedesired performance.

On the reverse link, the base station typically does not know a prioriwhich remote terminals have packet data to transmit, or how much. Thebase station is typically aware of each received remote terminal'sefficiency, which may be quantified by theenergy-per-bit-to-total-noise-plus-interface ratio, Ec/(No+Io), neededat the base station to correctly receive a data transmission. The basestation may then allocate resources to the remote terminals wheneverrequested and as available.

Because of uncertainty in user demands, the usage on the reverse linkmay fluctuate widely. If many remote terminals transmit at the sametime, high interference is generated at the base station. The transmitpower from the remote terminals would need to be increased to maintainthe target Ec/(No+Io), which would then result in higher levels ofinterference. If the transmit power is further increased in this manner,a “black out” may ultimately result and the transmissions from all or alarge percentage of the remote terminals may not be properly received.This is due to the remote terminal not being able to transmit atsufficient power to close the link to the base station.

In a CDMA system, the channel loading on the reverse link is oftencharacterized by what is referred to as the “rise-over-thermal”. Therise-over-thermal is the ratio of the total received power at a basestation receiver to the power of the thermal noise. Based on theoreticalcapacity calculations for a CDMA reverse link, there is a theoreticalcurve that shows the rise-over-thermal increasing with loading. Theloading at which the rise-over-thermal is infinite is often referred toas the “pole”. A loading that has a rise-over-thermal of 3 dBcorresponds to a loading of about 50%, or about half of the number ofusers that can be supported when at the pole. As the number of usersincreases and as the data rates of the users increase, the loadingbecomes higher. Correspondingly, as the loading increases, the amount ofpower that a remote terminal must transmit increases. Therise-over-thermal and channel loading are described in further detail byA. J. Viterbi in “CDMA: Principles of Spread Spectrum Communication,”Addison-Wesley Wireless Communications Series, May 1995, ISBN:0201633744, which is incorporated herein by reference.

The Viterbi reference provides classical equations that show therelationship between the rise-over-thermal, the number of users, and thedata rates of the users. The equations also show that there is greatercapacity (in bits/second) if a few users transmit at a high rate than alarger number of users transmit at a higher rate. This is due to theinterference between transmitting users.

In a typical CDMA system, many users' data rates are continuouslychanging. For example, in an IS-95 or cdma2000 system, a voice usertypically transmits at one of four rates, corresponding to the voiceactivity at the remote terminal, as described in U.S. Pat. Nos.5,657,420 and 5,778,338, both entitled “VARIABLE RATE VOCODER” and U.S.Pat. No. 5,742,734, entitled “ENCODING RATE SELECTION IN A VARIABLE RATEVOCODER”. Similarly, many data users are continually varying their datarates. All this creates a considerable amount of variation in the amountof data being transmitted simultaneously, and hence a considerablevariation in the rise-over-thermal.

As can be seen from the above, there is a need in the art for a reverselink channel structure capable of achieving high performance for packetdata transmission, and which takes into consideration the datatransmission characteristics of the reverse links.

SUMMARY

Aspects of the invention provide mechanisms that support effective andefficient allocation and utilization of the reverse link resources. Inone aspect, mechanisms are provided to quickly assign resources (e.g.,supplemental channels) as needed, and to quickly de-assign the resourceswhen not needed or to maintain system stability. The reverse linkresources may be quickly assigned and de-assigned via short messagesexchanged on control channels on the forward and reverse links. Inanother aspect, mechanisms are provided to facilitate efficient andreliable data transmission. In particular, a reliableacknowledgment/negative acknowledgment scheme and an efficientretransmission scheme are provided. In yet another aspect, mechanismsare provided to control the transmit power and/or data rate of theremote terminals to achieve high performance and avoid instability.Another aspect of the invention provides a channel structure capable ofimplementing the features described above. These and other aspects aredescribed in further detail below.

The disclosed embodiments further provide methods, channel structures,and apparatus that implement various aspects, embodiments, and featuresof the invention, as described in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a diagram of a wireless communication system that supports anumber of users;

FIG. 2 is a simplified block diagram of an embodiment of a base stationand a remote terminal;

FIGS. 3A and 3B are diagrams of a reverse and a forward channelstructure, respectively;

FIG. 4 is a diagram illustrating a communication between the remoteterminal and base station to assign a reverse link supplemental channel(R-SCH);

FIGS. 5A and 5B are diagrams illustrating a data transmission on thereverse link and an Ack/Nak message transmission for two differentscenarios;

FIGS. 6A and 6B are diagrams illustrating an acknowledgment sequencingwith short and long acknowledgment delays, respectively;

FIG. 7 is a flow diagram that illustrates a variable rate datatransmission on the R-SCH with fast congestion control, in accordancewith an embodiment of the invention; and

FIG. 8 is a diagram illustrating improvement that may be possible withfast control of the R-SCH.

DETAILED DESCRIPTION

FIG. 1 is a diagram of a wireless communication system 100 that supportsa number of users and capable of implementing various aspects of theinvention. System 100 provides communication for a number of cells, witheach cell being serviced by a corresponding base station 104. The basestations are also commonly referred to as base transceiver systems(BTSs). Various remote terminals 106 are dispersed throughout thesystem. Each remote terminal 106 may communicate with one or more basestations 104 on the forward and reverse links at any particular moment,depending on whether or not the remote terminal is active and whether ornot it is in soft handoff.

The forward link refers to transmission from base station 104 to remoteterminal 106, and the reverse link refers to transmission from remoteterminal 106 to base station 104. As shown in FIG. 1, base station 104 acommunicates with remote terminals 106 a, 106 b, 106 c, and 106 d, andbase station 104 b communicates with remote terminals 106 d, 106 e, and106 f. Remote terminal 106 d is in soft handoff and concurrentlycommunicates with base stations 104 a and 104 b.

In system 100, a base station controller (BSC) 102 couples to basestations 104 and may further couple to a public switched telephonenetwork (PSTN). The coupling to the PSTN is typically achieved via amobile switching center (MSC), which is not shown in FIG. 1 forsimplicity. The BSC may also couple into a packet network, which istypically achieved via a packet data serving node (PDSN) that is alsonot shown in FIG. 1. BSC 102 provides coordination and control for thebase stations coupled to it. BSC 102 further controls the routing oftelephone calls among remote terminals 106, and between remote terminals106 and users coupled to the PSTN (e.g., conventional telephones) and tothe packet network, via base stations 104.

System 100 may be designed to support one or more CDMA standards such as(1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standardfor Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95standard), (2) the “TIA/EIA-98-D Recommended Minimum Standard forDual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98standard), (3) the documents offered by a consortium named “3rdGeneration Partnership Project” (3GPP) and embodied in a set ofdocuments including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS25.213, and 3G TS 25.214 (the W-CDMA standard), (4) the documentsoffered by a consortium named “3rd Generation Partnership Project 2”(3GPP2) and embodied in a set of documents including Document Nos.C.S0002-A, C.S0005-A, C.S0010-A, C.S0011-A. C.S0024, and C.S0026 (thecdma2000 standard), and (5) some other standards. In the case of the3GPP and 3GPP2 documents, these are converted by standards bodiesworldwide (e.g., TIA, ETSI, ARIB, TTA, and CWTS) into regional standardsand have been converted into international standards by theInternational Telecommunications Union (ITU). These standards areincorporated herein by reference.

FIG. 2 is a simplified block diagram of an embodiment of base station104 and remote terminal 106, which are capable of implementing variousaspects of the invention. For a particular communication, voice data,packet data, and/or messages may be exchanged between base station 104and remote terminal 106. Various types of messages may be transmittedsuch as messages used to establish a communication session between thebase station and remote terminal and messages used to control a datatransmission (e.g., power control, data rate information,acknowledgment, and so on). Some of these message types are described infurther detail below.

For the reverse link, at remote terminal 106, voice and/or packet data(e.g., from a data source 210) and messages (e.g., from a controller230) are provided to a transmit (TX) data processor 212, which formatsand encodes the data and messages with one or more coding schemes togenerate coded data. Each coding scheme may include any combination ofcyclic redundancy check (CRC), convolutional, Turbo, block, and othercoding, or no coding at all. Typically, voice data, packet data, andmessages are coded using different schemes, and different types ofmessage may also be coded differently.

The coded data is then provided to a modulator (MOD) 214 and furtherprocessed (e.g., covered, spread with short PN sequences, and scrambledwith a long PN sequence assigned to the user terminal). The modulateddata is then provided to a transmitter unit (TMTR) 216 and conditioned(e.g., converted to one or more analog signals, amplified, filtered, andquadrature modulated) to generate a reverse link signal. The reverselink signal is routed through a duplexer (D) 218 and transmitted via anantenna 220 to base station 104.

At base station 104, the reverse link signal is received by an antenna250, routed through a duplexer 252, and provided to a receiver unit(RCVR) 254. Receiver unit 254 conditions (e.g., filters, amplifies,downconverts, and digitizes) the received signal and provides samples. Ademodulator (DEMOD) 256 receives and processes (e.g., despreads,decovers, and pilot demodulates) the samples to provide recoveredsymbols. Demodulator 256 may implement a rake receiver that processesmultiple instances of the received signal and generates combinedsymbols. A receive (RX) data processor 258 then decodes the symbols torecover the data and messages transmitted on the reverse link. Therecovered voice/packet data is provided to a data sink 260 and therecovered messages may be provided to a controller 270. The processingby demodulator 256 and RX data processor 258 are complementary to thatperformed at remote terminal 106. Demodulator 256 and RX data processor258 may further be operated to process multiple transmissions receivedvia multiple channels, e.g., a reverse fundamental channel (R-FCH) and areverse supplemental channel (R-SCH). Also, transmissions may bereceived simultaneously from multiple remote terminals, each of whichmay be transmitting on a reverse fundamental channel, a reversesupplemental channel, or both.

On the forward link, at base station 104, voice and/or packet data(e.g., from a data source 262) and messages (e.g., from controller 270)are processed (e.g., formatted and encoded) by a transmit (TX) dataprocessor 264, further processed (e.g., covered and spread) by amodulator (MOD) 266, and conditioned (e.g., converted to analog signals,amplified, filtered, and quadrature modulated) by a transmitter unit(TMTR) 268 to generate a forward link signal. The forward link signal isrouted through duplexer 252 and transmitted via antenna 250 to remoteterminal 106.

At remote terminal 106, the forward link signal is received by antenna220, routed through duplexer 218, and provided to a receiver unit 222.Receiver unit 222 conditions (e.g., downconverts, filters, amplifies,quadrature demodulates, and digitizes) the received signal and providessamples. The samples are processed (e.g., despreaded, decovered, andpilot demodulated) by a demodulator 224 to provide symbols, and thesymbols are further processed (e.g., decoded and checked) by a receivedata processor 226 to recover the data and messages transmitted on theforward link. The recovered data is provided to a data sink 228, and therecovered messages may be provided to controller 230.

The reverse link has some characteristics that are very different fromthose of the forward link. In particular, the data transmissioncharacteristics, soft handoff behaviors, and fading phenomenon aretypically very different between the forward and reverse links.

As noted above, on the reverse link, the base station typically does notknow a priori which remote terminals have packet data to transmit, orhow much. Thus, the base station may allocate resources to the remoteterminals whenever requested and as available. Because of uncertainty inuser demands, the usage on the reverse link may fluctuate widely.

In accordance with aspects of the invention, mechanisms are provided toeffectively and efficiently allocate and utilize the reverse linkresources. In one aspect, mechanisms are provided to quickly assignresources as needed, and to quickly de-assign resources when not neededor to maintain system stability. The reverse link resources may beassigned via a supplemental channel that is used for packet datatransmission. In another aspect, mechanisms are provided to facilitateefficient and reliable data transmission. In particular, a reliableacknowledgment scheme and an efficient retransmission scheme areprovided. In yet another aspect, mechanisms are provided to control thetransmit power of the remote terminals to achieve high performance andavoid instability. These and other aspects are described in furtherdetail below.

FIG. 3A is a diagram of an embodiment of a reverse channel structurecapable of implementing various aspects of the invention. In thisembodiment, the reverse channel structure includes an access channel, anenhanced access channel, a pilot channel (R-PICH), a common controlchannel (R-CCCH), a dedicated control channel (R-DCCH), a fundamentalchannel (R-FCH), supplemental channels (R-SCH), and a reverse rateindicator channel (R-RICH). Different, fewer, and/or additional channelsmay also be supported and are within the scope of the invention. Thesechannels may be implemented similar to those defined by the cdma2000standard. Features of some of these channels are described below.

For each communication (i.e., each call), a specific set of channelsthat may be used for the communication and their configurations aredefined by one of a number of radio configurations (RC). Each RC definesa specific transmission format, which is characterized by variousphysical layer parameters such as, for example, the transmission rates,modulation characteristics, spreading rate, and so on. The radioconfigurations may be similar to those defined for the cdma2000standard.

The reverse dedicated control channel (R-DCCH) is used to transmit userand signaling information (e.g., control information) to the basestation during a communication. The R-DCCH may be implemented similar tothe R-DCCH defined in the cdma2000 standard.

The reverse fundamental channel (R-FCH) is used to transmit user andsignaling information (e.g., voice data) to the base station during acommunication. The R-FCH may be implemented similar to the R-FCH definedin the cdma2000 standard.

The reverse supplemental channel (R-SCH) is used to transmit userinformation (e.g., packet data) to the base station during acommunication. The R-SCH is supported by some radio configurations(e.g., RC3 through RC11), and is assigned to the remote terminals asneeded and if available. In an embodiment, zero, one, or twosupplemental channels (i.e., R-SCH1 and R-SCH2) may be assigned to theremote terminal at any given moment. In an embodiment, the R-SCHsupports retransmission at the physical layer, and may utilize differentcoding schemes for the retransmission. For example, a retransmission mayuse a code rate of ½ for the original transmission. The same rate ½ codesymbols may be repeated for the retransmission. In an alternativeembodiment, the underlying code may be a rate ¼ code. The originaltransmission may use ½ of the symbols and the retransmission may use theother half of the symbols. If a third retransmission is done, it canrepeat one of the group of symbols, part of each group, a subset ofeither group, and other possible combinations of symbols.

R-SCH2 may be used in conjunction with R-SCH1 (e.g., for RC11). Inparticular, R-SCH2 may be used to provide a different quality of service(QoS). Also, Type II and III hybrid ARQ schemes may be used inconjunction with the R-SCH. Hybrid ARQ schemes are generally describedby S. B. Wicker in “Error Control System for Digital Communication andStorage,” Prentice-Hall, 1995, Chapter 15, which is incorporated hereinby reference. Hybrid ARQ schemes are also described in the cdma2000standard.

The reverse rate indicator channel (R-RICH) is used by the remoteterminal to provide information pertaining to the (packet) transmissionrate on one or more reverse supplemental channels. Table 1 lists thefields for a specific format of the R-RICH. In an embodiment, for eachdata frame transmission on the R-SCH, the remote terminal sends areverse rate indicator (RRI) symbol, which indicates the data rate forthe data frame. The remote terminal also sends the sequence number ofthe data frame being transmitted, and whether the data frame is a firsttransmission or a retransmission. Different, fewer, and/or additionalfields may also be used for the R-RICH and are within the scope of theinvention. The information in Table 1 is sent by the remote terminal foreach data frame transmitted on the supplemental channel (e.g., each 20msec).

TABLE 1 Field Length (bits) RRI 3 SEQUENCE_NUM 2 RETRAN_NUM 2

If there are multiple reverse supplemental channels (e.g., R-SCH1 andR-SCH2), then there can be multiple R-RICH channels (e.g., R-RICH 1 andR-RICH 2), each with the RRI, SEQUENCE_NUM, and RETRAN_NUM fields.Alternatively, the fields for multiple reverse supplemental channels maybe combined into a single R-RICH channel. In a particular embodiment,the RRI field is not used, and fixed transmission rates are used or thebase station performs blind rate determination in which the basedetermines the transmission rate from the data. Blind rate determinationmay be achieved in a manner described in U.S. Pat. No. 6,175,590,entitled “METHOD AND APPARATUS FOR DETERMINING THE RATE OF RECEIVED DATAIN A VARIABLE RATE COMMUNICATION SYSTEM,” issued Jan. 16, 2001, U.S.Pat. No. 5,751,725, entitled “METHOD AND APPARATUS FOR DETERMINING THERATE OF RECEIVED DATA IN A VARIABLE RATE COMMUNICATION SYSTEM,” issuedMay 12, 1998, both of which are assigned to the assignee of the presentapplication and incorporated herein by reference.

FIG. 3B is a diagram of an embodiment of a forward channel structurecapable of supporting various aspects of the invention. In thisembodiment, the forward channel structure includes common channels,pilot channels, and dedicated channels. The common channels include abroadcast channel (F-BCCH), a quick paging channel (F-QPCH), a commoncontrol channel (F-CCCH), and a common power control channel (F-CPCCH).The pilot channels include a basic pilot channel and an auxiliary pilotchannel. And the dedicated channels include a fundamental channel(F-FCH), a supplemental channel (F-SCH), a dedicated auxiliary channel(F-APICH), a dedicated control channel (F-DCCH), and a dedicated packetcontrol channel (F-CPDCCH). Again, different, fewer, and/or additionalchannels may also be supported and are within the scope of theinvention. These channels may be implemented similar to those defined bythe cdma2000 standard. Features of some of these channels are describedbelow.

The forward common power control channel (F-CPCCH) is used by the basestation to transmit power control subchannels (e.g., one bit persubchannel) for power control of the R-PICH, R-FCH, R-DCCH, and R-SCH.In an embodiment, upon channel assignment, a remote terminal is assigneda reverse link power control subchannel from one of three sources—theF-DCCH, F-SCH, and F-CPCCH. The F-CPCCH may be assigned if the reverselink power control subchannel is not provided from either the F-DCCH orF-SCH.

In an embodiment, the available bits in the F-CPCCH may be used to formone or more power control subchannels, which may then be assigned fordifferent uses. For example, a number of power control subchannels maybe defined and used for power control of a number of reverse linkchannels. Power control for multiple channels based on multiple powercontrol subchannels may be implemented as described in U.S. Pat. No.5,991,284, entitled “SUBCHANNEL POWER CONTROL,” issued Nov. 23, 1999,assigned to the assignee of the present application and incorporatedherein by reference.

In one specific implementation, an 800 bps power control subchannelcontrols the power of the reverse pilot channel (R-PICH). All reversetraffic channels (e.g., the R-FCH, R-DCCH, and R-SCH) have their powerlevels related to the R-PICH by a known relationship, e.g., as describedin C.S0002. The ratio between two channels is often referred to as thetraffic-to-pilot ratio. The traffic-to-pilot ratio (i.e., the powerlevel of the reverse traffic channel relative to the R-PICH) can beadjusted by messaging from the base station. However, this messaging isslow, so a 100 bits/second (bps) power control subchannel may be definedand used for power control of the R-SCH. In an embodiment, this R-SCHpower control subchannel controls the R-SCH relative to the R-PICH. Inanother embodiment, the R-SCH power control subchannel controls theabsolute transmission power of the R-SCH.

In an aspect of the invention, a “congestion” control subchannel mayalso be defined for control of the R-SCH, and this congestion controlsubchannel may be implemented based on the R-SCH power controlsubchannel or another subchannel.

Power control for the reverse link is described in further detail below.

The forward dedicated packet control channel (F-DPCCH) is used totransmit user and signaling information to a specific remote terminalduring a communication. The F-DPCCH may be used to control a reverselink packet data transmission. In an embodiment, the F-DPCCH is encodedand interleaved to enhance reliability, and may be implemented similarto the F-DCCH defined by the cdma2000 standard.

Table 2 lists the fields for a specific format of the F-DPCCH. In anembodiment, the F-DPCCH has a frame size of 48 bits, of which 16 areused for CRC, 8 bits are used for the encoder tail, and 24 bits areavailable for data and messaging. In an embodiment, the defaulttransmission rate for the F-DPCCH is 9600 bps, in which case a 48-bitframe can be transmitted in 5 msec time interval. In an embodiment, eachtransmission (i.e., each F-DPCCH frame) is covered with a public longcode of the recipient remote terminal to which the frame is targeted.This avoids the need to use an explicit address (hence, the channel isreferred to as a “dedicated” channel). However, the F-DPCCH is also“common” since a large number of remote terminals in dedicated channelmode may continually monitor the channel. If a message is directed to aparticular remote terminal and is received correctly, then the CRC willcheck.

TABLE 2 Field Number of Bits/Frame Information 24 Frame QualityIndicator 16 Encoder Tail 8

The F-DPCCH may be used to transmit mini-messages, such as the onesdefined by the cdma2000 standard. For example, the F-DPCCH may be usedto transmit a Reverse Supplemental Channel Assignment Mini Message(RSCAMM) used to grant the F-SCH to the remote terminal.

The forward common packet Ack/Nak channel (F-CPANCH) is used by the basestation to transmit (1) acknowledgments (Ack) and negativeacknowledgments (Nak) for a reverse link packet data transmission and(2) other control information. In an embodiment, acknowledgments andnegative acknowledgments are transmitted as n-bit Ack/Nak messages, witheach message being associated with a corresponding data frametransmitted on the reverse link. In an embodiment, each Ack/Nak messagemay include 1, 2, 3, or 4 bits (or possible more bits), with the numberof bits in the message being dependent on the number of reverse linkchannels in the service configuration. The n-bit Ack/Nak message may beblock coded to increase reliability or transmitted in the clear.

In an aspect, to improve reliability, the Ack/Nak message for aparticular data frame is retransmitted in a subsequent frame (e.g., 20msec later) to provide time diversity for the message. The timediversity provides additional reliability, or may allow for thereduction in power used to send the Ack/Nak message while maintainingthe same reliability. The Ack/Nak message may use error correctingcoding as is well known in the art. For the retransmission, the Ack/Nakmessage may repeat the exact same code word or may use incrementalredundancy. Transmission and retransmission of the Ack/Nak is describedin further detail below.

Several types of control are used on the forward link to control thereverse link. These include controls for supplemental channel requestand grant, Ack/Nak for a reverse link data transmission, power controlof the data transmission, and possibly others.

The reverse link may be operated to maintain the rise-over-thermal atthe base station relatively constant as long as there is reverse linkdata to be transmitted. Transmission on the R-SCH may be allocated invarious ways, two of which are described below:

-   -   By infinite allocation. This method is used for real-time        traffic that cannot tolerate much delay. The remote terminal is        allowed to transmit immediately up to a certain allocated data        rate.    -   By scheduling. The remote terminal sends an estimate of its        buffer size. The base station determines when the remote        terminal is allowed to transmit. This method is used for        available bit rate traffic. The goal of a scheduler is to limit        the number of simultaneous transmissions so that the number of        simultaneously transmitting remote terminals is limited, thus        reducing the interference between remote terminals.

Since channel loading can change relatively dramatically, a fast controlmechanism may be used to control the transmit power of the R-SCH (e.g.,relative to the reverse pilot channel), as described below.

A communication between the remote terminal and base station toestablish a connection may be achieved as follows. Initially, the remoteterminal is in a dormant mode or is monitoring the common channels withthe slotted timer active (i.e., the remote terminal is monitoring eachslot). At a particular time, the remote terminal desires a datatransmission and sends a short message to the base station requesting areconnection of the link. In response, the base station may send amessage specifying the parameters to be used for the communication andthe configurations of various channels. This information may be sent viaan Extended Channel Assignment Message (ECAM), a specially definedmessage, or some other message. This message may specify the following:

-   -   The MAC_ID for each member of the remote terminal's Active Set        or a subset of the Active Set. The MAC_ID is later used for        addressing on the forward link.    -   Whether the R-DCCH or R-FCH is used on the reverse link.    -   For the F-CPANCH, the spreading (e.g., Walsh) codes and Active        Set to be used. This may be achieved by (1) sending the        spreading codes in the ECAM, or (2) transmitting the spreading        codes in a broadcast message, which is received by the remote        terminal. The spreading codes of neighbor cells may need to be        included. If the same spreading codes can be used in neighboring        cells, only a single spreading code may need to be sent.    -   For the F-CPCCH, the Active Set, the channel identity, and the        bit positions. In an embodiment, the MAC_ID may be hashed to the        F-CPCCH bit positions to obviate the need to send the actual bit        positions or subchannel ID to the remote terminal. This hashing        is a pseudo-random method to map a MAC_ID to a subchannel on the        F-CPCCH. Since different simultaneous remote terminals are        assigned distinct MAC_IDs, the hashing can be such that these        MAC_IDs also map to distinct F-CPCCH subchannels. For example,        if there are K possible bit positions and N possible MAC_IDs,        then K=□N×((40503×KEY) mod 2¹⁶)/2¹⁶□, where KEY is the number        that is fixed in this instance. There are many other hash        functions that can be used and discussions of such can be found        in many textbooks dealing with computer algorithms.

In an embodiment, the message from the base station (e.g., the ECAM) isprovided with a specific field, USE_OLD_SERV_CONFIG, used to indicatewhether or not the parameters established in the last connection are tobe used for the reconnection. This field can be used to obviate the needto send the Service Connect Message upon reconnection, which may reducedelay in re-establishing the connection.

Once the remote terminal has initialized the dedicated channel, itcontinues, for example, as described in the cdma2000 standard.

As noted above, better utilization of the reverse link resources may beachieved if the resources can be quickly allocated as needed and ifavailable. In a wireless (and especially mobile) environment, the linkconditions continually fluctuate, and long delay in allocating resourcesmay result in inaccurate allocation and/or usage. Thus, in accordancewith an aspect of the invention, mechanisms are provided to quicklyassign and de-assign supplemental channels.

FIG. 4 is a diagram illustrating a communication between the remoteterminal and base station to assign and de-assign a reverse linksupplemental channel (R-SCH), in accordance with an embodiment of theinvention. The R-SCH may be quickly assigned and de-assigned as needed.When the remote terminal has packet data to send that requires usage ofthe R-SCH, it requests the R-SCH by sending to the base station aSupplemental Channel Request Mini Message (SCRMM) (step 412). The SCRMMis a 5 msec message that may be sent on the R-DCCH or R-FCH. The basestation receives the message and forwards it to the BSC (step 414). Therequest may or may not be granted. If the request is granted, the basestation receives the grant (step 416) and transmits the R-SCH grantusing a Reverse Supplemental Channel Assignment Mini Message (RSCAMM)(step 418). The RSCAMM is also a 5 msec message that may be sent on theF-FCH or F-DCCH (if allocated to the remote terminal) or on the F-DPCCH(otherwise). Once assigned, the remote terminal may thereafter transmiton the R-SCH (step 420).

Table 3 lists the fields for a specific format of the RSCAMM. In thisembodiment, the RSCAMM includes 8 bits of layer 2 fields (i.e., theMSG_TYPE, ACK_SEQ, MSG_SEQ, and ACK_REQUIREMENT fields), 14 bits oflayer 3 fields, and two reserved bits that are also used for padding asdescribed in C.S0004 and C.S0005. The layer 3 (i.e., signaling layer)may be as defined in the cdma2000 standard.

TABLE 3 Field Length (Bits) MSG_TYPE 3 ACK_SEQUENCE 2 MSG_SEQUENCE 2ACK_REQUIREMENT 1 REV_SCH_ID 1 REV_SCH_DURATION 4 REV_SCH_START_TIME 5REV_SCH_NUM_BITS_IDX 4 RESERVED 2

When the remote terminal no longer has data to send on the R-SCH, itsends a Resource Release Request Mini Message (RRRMM) to the basestation. If there is no additional signaling required between the remoteterminal and base station, the base station responds with an ExtendedRelease Mini Message (ERMM). The RRRMM and ERMM are also 5 msec messagesthat may be sent on the same channels used for sending the request andgrant, respectively.

There are many scheduling algorithms that may be used to schedule thereverse link transmissions of remote terminals. These algorithms maytradeoff between rates, capacity, delay, error rates, and fairness(which gives all users some minimal level of services), to indicate someof the main criteria. In addition, the reverse link is subject to thepower limitations of the remote terminal. In a single cell environment,the greatest capacity will exist when the smallest number of remoteterminals is allowed to transmit with the highest rate that the remoteterminal can support—both in terms of capability and the ability toprovide sufficient power. However, in a multiple cell environment, itmay be preferable for remote terminals near the boundary with anothercell to transmit at a lower rate. This is because their transmissionscause interference into multiple cells—not just a single cell. Anotheraspect that tends to maximize the reverse link capacity is to operate ahigh rise-over-thermal at the base station, which indicates high loadingon the reverse link. It is for this reason that aspects of the inventionuse scheduling. The scheduling attempts to have a few number of remoteterminals simultaneously transmit—those that do transmit are allowed totransmit at the highest rates that they can support.

However, a high rise-over-thermal tends to result in less stability asthe system is more sensitive to small changes in loading. It is for thisreason that fast scheduling and control is important. Fast scheduling isimportant because the channel conditions change quickly. For instance,fading and shadowing processes may result in a signal that was weaklyreceived at a base station suddenly becoming strong at the base station.For voice or certain data activity, the remote terminal autonomouslychanges the transmission rate While scheduling may be able to take someof this into account, scheduling may not be able to react sufficientlyfast enough. For this reason, aspects of the invention provide fastpower control techniques, which are described in further detail below.

An aspect of the invention provides a reliable acknowledgment/negativeacknowledgment scheme to facilitate efficient and reliable datatransmission. As described above, acknowledgments (Ack) and negativeacknowledgments (Nak) are sent by the base station for data transmissionon the R-SCH. The Ack/Nak can be sent using the F-CPANCH.

Table 4 shows a specific format for an Ack/Nak message. In this specificembodiment, the Ack/Nak message includes 4 bits that are assigned tofour reverse link channels—the R-FCH, R-DCCH, R-SCH1, and R-SCH2. In anembodiment, an acknowledgment is represented by a bit value of zero(“0”) and a negative acknowledgment is represented by a bit value of one(“1”). Other Ack/Nak message formats may also be used and are within thescope of the invention.

TABLE 4 R-FCH, All Channels R-DCCH, and R-FCH and Used R-SCH1 UsedR-DCCH Used Number_Type Number_Type Number_Type Description (binary)(binary) (binary) ACK_R-FCH xxx0 xxx0 xx00 NAK_R-FCH xxx1 xxx1 xx11ACK_R-DCCH xx0x xx0x — NAK_R-DCCH xx1x xx1x — ACK_R-SCH1 x0xx 00xx 00xxNAK_R-SCH1 x1xx 11xx 11xx ACK_R-SCH2 0xxx — — NAK_R-SCH2 1xxx — —

In an embodiment, the Ack/Nak message is sent block coded but a CRC isnot used to check for errors. This keeps the Ack/Nak message short andfurther allows the message to be sent with a small amount of energy.However, no coding may also be used for the Ack/Nak message, or a CRCmay be attached to the message, and these variations are within thescope of the invention. In an embodiment, the base station sends anAck/Nak message corresponding to each frame in which the remote terminalhas been given permission to transmit on the R-SCH, and does not sendAck/Nak messages during frames that the remote terminal is not givenpermission to transmit.

During a packet data transmission, the remote terminal monitors theF-CPANCH for Ack/Nak messages that indicate the results of thetransmission. The Ack/Nak messages may be transmitted from any number ofbase stations in the remote terminal's Active Set (e.g., from one or allbase stations in the Active Set). The remote terminal can performdifferent actions depending on the received Ack/Nak messages. Some ofthese actions are described below.

If an Ack is received by the remote terminal, the data framecorresponding to the Ack may be removed from the remote terminal'sphysical layer transmit buffer (e.g., data source 210 in FIG. 2) sincethe data frame was correctly received by the base station.

If a Nak is received by the remote terminal, the data framecorresponding to the Nak may be retransmitted by the remote terminal ifit is still in the physical layer transmit buffer. In an embodiment,there is a one-to-one correspondence between a forward link Ack/Nakmessage and a transmitted reverse link data frame. The remote terminalis thus able to identify the sequence number of the data frame notreceived correctly by the base station (i.e., the erased frame) based onthe frame in which the Nak was received. If this data frame has not beendiscarded by the remote terminal, it may be retransmitted at the nextavailable time interval, which is typically the next frame.

If neither an Ack nor a Nak was received, there are several nextpossible actions for the remote terminal. In one possible action, thedata frame is maintained in the physical layer transmit buffer andretransmitted. If the retransmitted data frame is then correctlyreceived at the base station, then the base station transmits an Ack.Upon correct receipt of this Ack, the remote terminal discards the dataframe. This would be the best approach if the base station did notreceive the reverse link transmission.

Another possible action is for the remote terminal to discard the dataframe if neither an Ack nor a Nak was received. This would be the bestalternative if the base station had received the frame but the Acktransmission was not received by the remote terminal. However, theremote terminal does not know the scenario that occurred and a policyneeds to be chosen. One policy would be to ascertain the likelihood ofthe two events happening and performing the action that maximizes thesystem throughput.

In an embodiment, each Ack/Nak message is retransmitted a particulartime later (e.g., at the next frame) to improve reliability of theAck/Nak. Thus, if neither an Ack nor a Nak was received, the remoteterminal combines the retransmitted Ack/Nak with the original Ack/Nak.Then, the remote terminal can proceed as described above. And if thecombined Ack/Nak still does not result in a valid Ack or Nak, the remoteterminal may discard the data frame and continue to transmit the nextdata frame in the sequence. The second transmission of the Ack/Nak maybe at the same or lower power level relative to that of the firsttransmission.

If the base station did not actually receive the data frame afterretransmissions, then a higher signaling layer at the base station maygenerate a message (e.g., an RLP NAK), which may result in theretransmission of the entire sequence of data frames that includes theerased frame.

FIG. 5A is a diagram illustrating a data transmission on the reverselink (e.g., the R-SCH) and an Ack/Nak transmission on the forward link.The remote terminal initially transmits a data frame, in frame k, on thereverse link (step 512). The base station receives and processes thedata frame, and provides the demodulated frame to the BSC (step 514). Ifthe remote terminal is in soft handoff, the BSC may also receivedemodulated frames for the remote terminal from other base stations.

Based on the received demodulated frames, the BSC generates an Ack or aNak for the data frame. The BSC then sends the Ack/Nak to the basestation(s) (step 516), which then transmit the Ack/Nak to the remoteterminal during frame k+1 (step 518). The Ack/Nak may be transmittedfrom one base station (e.g., the best base station) or from a numberbase stations in the remote terminal's Active Set. The remote terminalreceives the Ack/Nak during frame k+1. If a Nak is received, the remoteterminal retransmits the erased frame at the next available transmissiontime, which in this example is frame k+2 (step 520). Otherwise, theremote terminal transmits the next data frame in the sequence.

FIG. 5B is a diagram illustrating a data transmission on the reverselink and a second transmission of the Ack/Nak message. The remoteterminal initially transmits a data frame, in frame k, on the reverselink (step 532). The base station receives and processes the data frame,and provides the demodulated frame to the BSC (step 534). Again, forsoft handoff, the BSC may receive other demodulated frames for theremote terminal from other base stations.

Based on the received demodulated frames, the BSC generates an Ack or aNak for the frame. The BSC then sends the Ack/Nak to the base station(s)(step 536), which then transmit the Ack/Nak to the remote terminalduring frame k+l (step 538). In this example, the remote terminal doesnot receive the Ack/Nak transmitted during frame k+1. However, theAck/Nak for the data frame transmitted in frame k is transmitted asecond time during frame k+2, and is received by the remote terminal(step 540). If a Nak is received, the remote terminal retransmits theerased frame at the next available transmission time, which in thisexample is frame k+3 (step 542). Otherwise, the remote terminaltransmits the next data frame in the sequence. As shown in FIG. 5B, thesecond transmission of the Ack/Nak improves the reliability of thefeedback, and can result in improved performance for the reverse link.

In an alternative embodiment, the data frames are not sent back to theBSC from the base station, and the Ack/Nak is generated from the basestation.

FIG. 6A is a diagram illustrating an acknowledgment sequencing withshort acknowledgment delay. The remote terminal initially transmits adata frame with a sequence number of zero, in frame k, on the reverselink (step 612). For this example, the data frame is received in errorat the base station, which then sends a Nak during frame k+1 (step 614).The remote terminal also monitors the F-CPANCH for an Ack/Nak messagefor each data frame transmitted on the reverse link. The remote terminalcontinues to transmit a data frame with a sequence number of one inframe k+1 (step 616).

Upon receiving the Nak in frame k+1, the remote terminal retransmits theerased frame with the sequence number of zero, in frame k+2 (step 618).The data frame transmitted in frame k+1 was received correctly, asindicated by an Ack received during frame k+2, and the remote terminaltransmits a data frame with a sequence number of two in frame k+3 (step620). Similarly, the data frame transmitted in frame k+2 was receivedcorrectly, as indicated by an Ack received during frame k+3, and theremote terminal transmits a data frame with a sequence number of threein frame k+4 (step 622). In frame k+5, the remote terminal transmits adata frame with a sequence number of zero for a new packet (step 624).

FIG. 6B is a diagram illustrating an acknowledgment sequencing with longacknowledgment delay such as when the remote terminal demodulates theAck/Nak transmission based upon the retransmission of the Ack/Nak asdescribed above. The remote terminal initially transmits a data framewith a sequence number of zero, in frame k, on the reverse link (step632). The data frame is received in error at the base station, whichthen sends a Nak (step 634). For this example, because of the longerprocessing delay, the Nak for frame k is transmitted during frame k+2.The remote terminal continues to transmit a data frame with a sequencenumber of one in frame k+1 (step 636) and a data frame with a sequencenumber of two in frame k+2 (step 638).

For this example, the remote terminal receives the Nak in frame k+2, butis not able to retransmit the erased frame at the next transmissioninterval. Instead, the remote terminal transmits a data frame with asequence number of three in frame k+3 (step 640). At frame k+4, theremote terminal retransmits the erased frame with the sequence number ofzero (step 642) since this frame is still in the physical layer buffer.Alternatively, the retransmission may be in frame k+3. And since thedata frame transmitted in frame k+1 was received correctly, as indicatedby an Ack received during frame k+3, and the remote terminal transmits adata frame with a sequence number of zero for a new packet (step 644).

As shown in FIG. 6B, the erased frame may be retransmitted at any timeas long as it is still available in the buffer and there is no ambiguityas to which higher layer packet the data frame belongs to. The longerdelay for the retransmission may be due to any number of reasons such as(1) longer delay to process and transmit the Nak, (2) non-detection ofthe first transmission of the Nak, (3) longer delay to retransmit theerased frame, and others.

An efficient and reliable Ack/Nak scheme can improve the utilization ofthe reverse link. A reliable Ack/Nak scheme may also allow data framesto be transmitted at lower transmit power. For example, withoutretransmission, a data frame needs to be transmitted at a higher powerlevel (P₁) required to achieve one percent frame error rate (1% FER). Ifretransmission is used and is reliable, a data frame may be transmittedat a lower power level (P₂) required to achieve 10% FER. The 10% erasedframes may be retransmitted to achieve an overall 1% FER for thetransmission. Typically, 1.1·P₂<P₁, and less transmit power is used fora transmission using the retransmission scheme. Moreover, retransmissionprovides time diversity, which may improve performance. Theretransmitted frame may also be combined with the first transmission ofthe frame at the base station, and the combined power from the twotransmissions may also improve performance. The recombining may allow anerased frame to be retransmitted at a lower power level.

An aspect of the invention provides various power control schemes forthe reverse link. In an embodiment, reverse link power control issupported for the R-FCH, R-SCH, and R-DCCH. This can be achieved via a(e.g., 800 bps) power control channel, which may be partitioned into anumber of power control subchannels. For example, a 100 bps powercontrol subchannel may be defined and used for the R-SCH. If the remoteterminal has not been allocated a F-FCH or F-DCCH, then the F-CPCCH maybe used to send power control bits to the remote terminal.

In one implementation, the (e.g., 800 bps) power control channel is usedto adjust the transmit power of the reverse link pilot. The transmitpower of the other channels (e.g., the R-FCH) is set relative to that ofthe pilot (i.e., by a particular delta). Thus, the transmit power forall reverse link channels may be adjusted along with the pilot. Thedelta for each non-pilot channel may be adjusted by signaling. Thisimplementation does not provide flexibility to quickly adjust thetransmit power of different channels.

In one embodiment, the forward common power control channel (F-CPCCH)may be used to form one or more power control subchannels that may thenbe used for various purposes. Each power control subchannel may bedefined using a number of available bits in the F-CPCCH (e.g., them^(th) bit in each frame). For example, some of the available bits inthe F-CPCCH may be allocated for a 100 bps power control subchannel forthe R-SCH. This R-SCH power control subchannel may be assigned to theremote terminal during channel assignment. The R-SCH power controlsubchannel may then be used to (more quickly) adjust the transmit powerof the designated R-SCH, e.g., relative to that of the pilot channel.For a remote terminal in soft handoff, the R-SCH power control may bebased on the OR-of-the-downs rule, which decreases the transmit power ifany base station in the remote terminal's Active Set directs a decrease.Since the power control is maintained at the base station, this permitsthe base station to adjust the transmitted power with minimal amount ofdelay and thus adjust the loading on the channel.

The R-SCH power control subchannel may be used in various manners tocontrol the transmission on the R-SCH. In an embodiment, the R-SCH powercontrol subchannel may be used to direct the remote terminal to adjustthe transmit power on the R-SCH by a particular amount (e.g., 1 dB, 2dB, or some other value). In another embodiment, the subchannel may beused to direct the remote terminal to reduce or increase transmit powerby a large step (e.g., 3 dB, or possibly more). In both embodiments, theadjustment in transmit power may be relative to the pilot transmitpower. In another embodiment, the subchannel may be directed to adjustthe data rate allocated to the remote terminal (e.g., to the next higheror lower rate). In yet another embodiment, the subchannel may be used todirect the remote terminal to temporarily stop transmission. And in yetanother embodiment, the remote terminal may apply different processing(e.g., different interleaving interval, different coding, and so on)based on the power control command. The R-SCH power control subchannelmay also be partitioned into a number of “sub-subchannels”, each ofwhich may be used in any of the manners described above. Thesub-subchannels may have the same or different bit rates. The remoteterminal may apply the power control immediately upon receiving thecommand, or may apply the command at the next frame boundary.

The ability to reduce the R-SCH transmit power by a large amount (ordown to zero) without terminating the communication session isespecially advantageous to achieve better utilization of the reverselink. Temporary reduction or suspension of a packet data transmissioncan typically be tolerated by the remote terminal. These power controlschemes can be advantageously used to reduce interference from a highrate remote terminal.

Power control of the R-SCH may be achieved in various manners. In oneembodiment, a base station monitors the received power from the remoteterminals with a power meter. The base station may even be able todetermine the amount of power received from each channel (e.g., theR-FCH, R-DCCH, R-SCH, and so on). The base station is also able todetermine the interference, some of which may be contributed by remoteterminals not being served by this base station. Based on the collectedinformation, the base station may adjust the transmit power of some orall remote terminals based on various factors. For example, the powercontrol may be based on the remote terminals' category of service,recent performance, recent throughput, and so on. The power control isperformed in a manner to achieve the desired system goals.

Power control may be implemented in various manners. Exampleimplementations are described in U.S. Pat. No. 5,485,486, entitled“METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMACELLULAR MOBILE TELEPHONE SYSTEM,” issued Jan. 16, 1996, U.S Pat. No.5,822,318, entitled “METHOD AND APPARATUS FOR CONTROLLING POWER IN AVARIABLE RATE COMMUNICATION SYSTEM,” issued Oct. 13, 1998, and U.S. Pat.No. 6,137,840, entitled “METHOD AND APPARATUS FOR PERFORMING FAST POWERCONTROL IN A MOBILE COMMUNICATION SYSTEM,” issued Oct. 24, 2000, allassigned to the assignee of the present application and incorporatedherein by reference.

In a typical method of power control that is used to control the levelof the R-PICH channel, the base station measures the level of theR-PICH, compares it to a threshold, and then determines whether toincrease or decrease the power of the remote terminal. The base stationtransmits a bit to the remote terminal instructing it to increase ordecrease its output power. If the bit is received in error, the remoteterminal will transmit at the incorrect power. During the nextmeasurement of the R-PICH level received by the base station, the basestation will determine that the received level is not at the desiredlevel and send a bit to the remote terminal to change its transmitpower. Thus, bit errors do not accumulate and the loop controlling theremote terminal's transmit power will stabilize to the correct value.

Errors in the bits sent to the remote terminal to control thetraffic-to-pilot ratio for congestion power control can cause thetraffic-to-pilot ratio to be other than that desired. However, the basestation typically monitors the level of the R-PICH for reverse powercontrol or for channel estimation. The base station can also monitor thelevel of the received R-SCH. By taking the ratio of the R-SCH level tothe R-PICH level, the base station can estimate the traffic-to-pilotratio in use by the remote terminal. If the traffic-to-pilot ratio isnot that which is desired, then the base station can set the bit thatcontrols the traffic-to-pilot ratio to correct for the discrepancy.Thus, there is a self-correction for bit errors.

Once a remote terminal has received a grant for the R-SCH, the remoteterminal typically transmits at the granted rate (or below in case itdoesn't have enough data to send or does not have sufficient power) forthe duration of the grant. The channel load from other remote terminalscan vary quite quickly as a result of fading and the like. As such, itmay be difficult for the base station to estimate the loading preciselyin advance.

In an embodiment, a “congestion” power control subchannel may beprovided to control a group of remote terminals in the same manner. Inthis case, instead of a single remote terminal monitoring the powercontrol subchannel to control the R-SCH, a group of remote terminalsmonitor the control subchannel. This power control subchannel can be at100 bps or at any other transmission rate. In one embodiment, thecongestion control subchannel is implemented with the power controlsubchannel used for the R-SCH. In another embodiment, the congestioncontrol subchannel is implemented as a “sub-subchannel” of the R-SCHpower control subchannel. In yet another embodiment, the congestioncontrol subchannel is implemented as a subchannel different from theR-SCH power control subchannel. Other implementations of the congestioncontrol subchannel may also be contemplated and are within the scope ofthe invention.

The remote terminals in the group may have the same category service(e.g., remote terminals having low priority available bit rate services)and may be assigned to a single power control bit per base station. Thisgroup control based on a single power control stream performs similar tothat directed to a single remote terminal to provide for congestioncontrol on the reverse link. In case of capacity overload, the basestation may direct this group of remote terminals to reduce theirtransmit power or their data rates, or to temporarily stop transmitting,based on a single control command. The reduction in the R-SCH transmitpower in response to the congestion control command may be a largedownward step relative to the transmit power of the pilot channel.

The advantage of a power control stream going to a group of remoteterminals instead of a single remote terminal is that less overheadpower is required on the forward link to support the power controlstream. It should be noted that the transmit power of a bit in the powercontrol stream can be equal to the power of the normal power controlstream used to the control the pilot channel for the remote terminalthat requires the most power. That is, the base station can determinethe remote terminal in the group that requires the greatest power in itsnormal power control stream and then use this power to transmit thepower control bit used for congestion control.

FIG. 7 is a flow diagram that illustrates a variable rate datatransmission on the R-SCH with fast congestion control, in accordancewith an embodiment of the invention. During the transmission on theR-SCH, the remote terminal transmits in accordance with the data rategranted in the Reverse Supplemental Channel Assignment Mini Message(RSAMM). If variable rate operation is permitted on the R-SCH, theremote terminal may transmit at any one of a number of permitted datarates.

If the remote terminal's R-SCH has been assigned to a congestion controlsubchannel, then, in an embodiment, the remote terminal adjusts thetraffic-to-pilot ratio based upon the bits received in the congestioncontrol subchannel. If variable rate operation is permitted on theR-SCH, the remote terminal checks the current traffic-to-pilot ratio. Ifit is below the level for a lower data rate, then the remote terminalreduces its transmission rate to the lower rate. If it is equal to orabove the level for a higher data rate, then the remote terminalincreases its transmission rate to the higher rate if it has sufficientdata to send.

Prior to the start of each frame, the remote terminal determines therate to use for transmitting the next data frame. Initially, the remoteterminal determines whether the R-SCH traffic-to-pilot ratio is belowthat for the next lower rate plus a margin Δ_(low), at step 712. If theanswer is yes, a determination is made whether the service configurationallows for a reduction in the data rate, at step 714. And if the answeris also yes, the data rate is decreased, and the same traffic-to-pilotratio is used, at step 716. And if the service configuration does notallow for a rate reduction, a particular embodiment would permit theremote terminal to temporarily stop transmitting.

Back at step 712, if the R-SCH traffic-to-pilot ratio is not above thatfor the next lower data rate plus the margin Δ_(low), a determination isnext made as to whether the R-SCH traffic-to-pilot ratio is greater thanthat for the next higher data rate minus a margin Δ_(high), at step 718.If the answer is yes, a determination is made whether the serviceconfiguration allows for an increase in the data rate, at step 720. Andif the answer is also yes, the transmission rate is increased, and thesame traffic-to-pilot ratio is used, at step 722. And if the serviceconfiguration does not allow for a rate increase, the remote terminaltransmits at the current rate.

FIG. 8 is a diagram illustrating improvement that may be possible withfast control of the R-SCH. On the left frame, without any fast controlof the R-SCH, the rise-over-thermal at the base station varies morewidely, exceeding the desired rise-over-thermal level by a larger amountin some instances (which may result in performance degradation for thedata transmissions from the remote terminals), and falling under desiredrise-over-thermal level by a larger amount in some other instances(resulting in under-utilization of the reverse link resources). Incontrast, on the right frame, with fast control of the R-SCH, therise-over-thermal at the base station is maintained more closely to thedesired rise-over-thermal level, which results in improved reverse linkutilization and performance.

In an embodiment, a base station may schedule more than one remoteterminal (via SCAM or ESCAM) to transmit, in response to receivingmultiple requests (via SCRM or SCRMM) from different remote terminals.The granted remote terminals may thereafter transmit on the R-SCH. Ifoverloading is detected at the base station, a “fast reduce” bit streammay be used to turn off (i.e., disable) a set of remote terminals (e.g.,all except one remote terminal). Alternatively, the fast reduce bitstream may be used to reduce the data rates of the remote terminals(e.g., by half). Temporarily disabling or reducing the data rates on theR-SCH for a number of remote terminals may be used for congestioncontrol, as described in further detail below. The fast reducecapability may also be advantageously used to shorten the schedulingdelay.

When the remote terminals are not in soft handoff with other basestations, the decision on which remote terminal is the most advantaged(efficient) to use the reverse link capacity may be made at the BTS. Themost efficient remote terminal may then be allowed to transmit while theothers are temporarily disabled. If the remote terminal signals the endof its available data, or possibly when some other remote terminalbecomes more efficient, the active remote terminal can quickly bechanged. These schemes may increase the throughput of the reverse link.

In contrast, for a usual set up in a cdma2000 system, a R-SCHtransmission can only start or stop via layer 3 messaging, which maytake several frames from composing to decoding at the remote terminal toget across. This longer delay causes a scheduler (e.g., at the basestation or BSC) to work with (1) less reliable, longer-term predictionsabout the efficiency of the remote terminal's channel condition (e.g.,the reverse link target pilot Ec/(No+Io) or set point), or (2) gaps inthe reverse link utilization when a remote terminal notifies the basestation of the end of its data (a common occurrence since a remoteterminal often claims it has a large amount of data to send to the basestation when requesting the R-SCH).

Referring back to FIG. 2, the elements of remote terminal 106 and basestation 104 may be designed to implement various aspects of theinvention, as described above. The elements of the remote terminal orbase station may be implemented with a digital signal processor (DSP),an application specific integrated circuit (ASIC), a processor, amicroprocessor, a controller, a microcontroller, a field programmablegate array (FPGA), a programmable logic device, other electronic units,or any combination thereof. Some of the functions and processingdescribed herein may also be implemented with software executed on aprocessor, such as controller 230 or 270.

Headings are used herein to serve as general indications of thematerials being disclosed, and are not intended to be construed as toscope.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method for transmitting data on a reverse link of a wirelesscommunication system by a remote terminal, comprising: selecting, by theremote terminal, a transmission data rate by comparing a currenttraffic-to-pilot ratio to one or more threshold levels, the remoteterminal selecting a lower data rate if the current traffic-to-pilotratio is less than a level corresponding to the lower data rate or ahigher data rate if the current traffic-to-pilot ratio is greater than alevel corresponding to the higher data rate; transmitting a frame ofdata on the reverse link via a data channel and according to theselected data rate; temporarily retaining the data frame in a buffer;monitoring for a message on a forward link indicating a received statusof the transmitted data frame; and processing the data frame based onthe received message.
 2. The method of claim 1, wherein the processingincludes; retransmitting the data frame if the message indicates thatthe transmitted data frame was incorrectly received.
 3. The method ofclaim 1, wherein the processing includes; discarding the data frame fromthe buffer if the message indicates that the transmitted data frame wascorrectly received.
 4. The method of claim 1, wherein the processingincludes; retaining the data frame in the buffer if the message is notproperly detected.
 5. The method of claim 1, further comprising:monitoring for a second transmission of the message; wherein theprocessing of the data frame is based on one or more received messagesfor the data frame.
 6. The method of claim 5, further comprising:combining the received messages for the data frame to provide a morereliable message.
 7. The method of claim 1, further comprising:identifying the transmitted data frame with a sequence number.
 8. Themethod of claim 7, further comprising: transmitting the sequence numberof the transmitted data frame via a signaling channel.
 9. The method ofclaim 1, further comprising: identifying the transmitted data frame aseither a first transmission or a retransmission.
 10. The method of claim1, wherein the current traffic-to-pilot ratio is determined dynamically.11. The method of claim 1, further comprising: transmitting a secondframe of data on the reverse link via the data channel and according toa second data rate, different from the first data rate, that is theother of the lower data rate or the higher data rate based on changes tothe current traffic-to-pilot ratio; temporarily retaining the seconddata frame in the buffer; monitoring for a second message on the forwardlink indicating a received status of the transmitted second data frame;and processing the second data frame based on the received secondmessage.
 12. A method for transmitting data on a reverse link of awireless communication system by a remote terminal, comprising:selecting, by the remote terminal, a transmission data rate by comparinga current traffic-to-pilot ratio to one or more threshold levels, theremote terminal selecting a lower data rate if the currenttraffic-to-pilot ratio is less than a level corresponding to the lowerdata rate or a higher data rate if the current traffic-to-pilot ratio isgreater than a level corresponding to the higher data rate; transmittinga frame of data on the reverse link via a data channel and according tothe selected data rate; temporarily retaining the data frame in abuffer; monitoring for a message on a forward link indicating a receivedstatus of the transmitted data frame; retransmitting the data frame ifthe message indicates that the transmitted data frame was incorrectlyreceived; discarding the data frame from the buffer if the messageindicates that the transmitted data frame was correctly received; andretaining the data frame in the buffer if the message is not properlydetected.
 13. A remote terminal for transmitting data on a reverse linkof a wireless communication system, comprising: means for selecting, bythe remote terminal, a transmission data rate by comparing a currenttraffic-to-pilot ratio to one or more threshold levels, the remoteterminal selecting a lower data rate if the current traffic-to-pilotratio is less than a level corresponding to the lower data rate or ahigher data rate if the current traffic-to-pilot ratio is greater than alevel corresponding to the higher data rate; means for transmitting aframe of data on the reverse link via a data channel and according tothe selected data rate; means for temporarily retaining the data framein a buffer; means for monitoring for a message on a forward linkindicating a received status of the transmitted data frame; and meansfor processing the data frame based on the received message.
 14. Aremote terminal, comprising: a processor for selecting, by the remoteterminal, a transmission data rate by comparing a currenttraffic-to-pilot ratio to one or more threshold levels, the remoteterminal selecting a lower data rate if the current traffic-to-pilotratio is less than a level corresponding to the lower data rate or ahigher data rate if the current traffic-to-pilot ratio is greater than alevel corresponding to the higher data rate; a transmitter fortransmitting in a wireless communication system a frame of data on areverse link via a data channel and according to the selected data rate;a buffer for temporarily retaining the data frame therein; a receiverfor monitoring for a message on a forward link indicating a receivedstatus of the transmitted data frame; and a controller for processingthe data frame based on the received message.